1. Field of the Invention
The present invention is directed to a method and apparatus suitable for measuring the position, shape, size and intensity distribution of the effective focal spot of an x-ray tube, particularly a computed tomography (CT) x-ray tube.
2. Description of the Prior Art
The stability of the position, size and intensity of the focal spot of an x-ray tube in a CT apparatus depends on many factors, including the mechanical tolerances of the anode surface and its dynamic stability, as well as the precision with which the electron beam, and its intensity distributions on the anode surface, can be controlled. The combined effects of tolerance deviations and electron beam divergence produce an electronic focal spot (hot spot) on the anode surface that varies over time with regard to one or more of the aforementioned characteristics. The position and intensity distribution of the hot spot yield the modulation transfer function (MTF), which is a fundamental measure of the imaging properties of the focal spot. Usually, the effective size of the focal spot is defined as the projection of the hot spot in the imaging direction perpendicular to the tube axis.
A newer type of computed tomography x-ray tube provides for active toggling of the focal spot position (flying focal spot system) to increase the sampling density. These tubes use magnetic deflection of the electron beam to toggle the focal spot position. Another newer type of CT x-ray tube, which is a rotating bulb tube wherein the bulb is piston-shaped, uses active magnetic deflection of the electron beam to bend the electron beam and keep it on the hot track. In x-ray tubes of this type, the effective size and shape of the focal spot depend additionally on the homogeneity and stability of the magnetic field used to deflect the electron beam.
In view of the multitude of factors which influence the aforementioned characteristics of the focal spot, there is a need for a simple apparatus to measure the size and shape of the effective focal spot, as well as its intensity distribution. It would be desirable to be able to make static or dynamic corrections, as needed, based on the information obtained with respect to the shape, size and intensity distribution.
A conventional approach employed for measuring the size of the focal spot is based on the generation of a pinhole camera-type image, as described in xe2x80x9cImaging Systems For Medical Diagnostics,xe2x80x9d Krestel, Ed., Siemens A G (1990), pages 230-231. This known technique is inappropriate, however, for measurements of the intensity distribution, because it is unable to correct for the inherent aperture errors which arise due to the finite diameter of the hole in the collimator through which the x-ray beam passes. Moreover, additional errors arise due to parasitic radiation, which may pass through the collimator material because the collimator may not be thick enough to completely attenuate such parasitic radiation. Consequently, this known technique has disadvantages associated therewith with regard to obtaining a good compromise between the hole diameter, the collimator thickness, and the total errors.
For a more exact analysis of the focal spot, at least with respect to its intensity distribution, other known techniques employ passing the x-ray beam through narrow slits oriented along the two major axes of the focal spot. The images obtained in this manner are analyzed with a photometer so as to measure the intensity distribution of the x-rays in the focal spot. This is also described in the aforementioned Imaging Systems For Medical Diagnostics text. This technique, however, is not appropriate for use to dynamically measure the shape and position of a focal spot which change over time such as, for example, because of rotation of the anode. Moreover, measurements obtained in this known manner cannot be used in the context of an automatic adjustment procedure for actively modifying the size and shape of the focal spot such as during a calibration phase.
It is an object of the present invention to provide a method and an apparatus for measuring the position, shape, size and intensity distribution of the effective focal spot of an x-ray tube which avoids, or at least minimizes, the aforementioned errors associated with known techniques and devices, and which is suited for use for automatically modifying one or more of the aforementioned parameters in an adjustment procedure.
The above object is achieved in accordance with the principles of the present invention in a method and apparatus for dynamically measuring the position, shape, size and intensity distribution of an effective focal spot of an x-ray tube, wherein at least one of the aforementioned characteristics changes with respect to time, wherein a detector array is provided that is composed of a number of detector elements, each of which generates an electrical signal dependent on x-rays incident thereon, and wherein a micro-hole collimator is disposed between the focal spot and the detector array at a location closer to the focal spot than to the detector array. When an x-ray beam is emitted from the focal spot, it passes through the aperture in the micro-hole collimator and a projection of the focal spot is produced on the array. Since the collimator is located closer to the focal spot than to the detector array, a magnification factor or zoom factor is achieved, which is defined by the ratio of the distance between the collimator and the detector array, and the distance between the focal spot and the collimator. Since the collimator is located closer to the focal spot than to the detector array, this magnification factor is greater than 1. Each of the detector elements of the detector array emits an electrical signal corresponding to the radiation incident thereon.
Since the respective output signals of the elements of the detector array are dependent on the intensity of the radiation incident on the detector element, the intensity distribution of the projected image, and thus of the focal spot, can be determined along any selected direction. The number of completely irradiated elements of the detector array, plus the respective detector elements which are partially irradiated which are located at the periphery of the projection, indicate the size and shape of the projected image, and thus of the focal spot. Since the output signals from the respective detector elements can be monitored over time, changes in these output signals, which in turn indicate changes in one or more of the aforementioned characteristics of the focal spot, can be easily monitored and identified.